Dental polishing agent and method of producing same



United States Patent 0.

DENTAL POLISHING AGENT AND METHOD OF PRODUCING SAME Henry V. Moss,Anniston, Ala., and Theodore W. Schilb, Centerville, Ohio, assignors toMonsanto Chemical Company, St. Louis, Mo., a corporation of Delaware NoDrawing. Application April 26, 1952, Serial No. 284,646

g] 9 Claims. (Cl. 23109) This invention relates to an improved dentalpolishing agent for use in the compounding of tooth paste or toothpowder. The dental polishing agent herein contemplated is an improvedform of anhydrous dicalcium phosphate.

According to the present requirements of the trade, anhydrous dicalciumphosphate to be useful as a dental polishing agent should have apolishing power of from about 2 to about 15 and preferably from about 10to about 13 by the scale hereinafter described. Moreover, it should meetthe above polishing power requirements and at the same time have animproved cleaning and stain-removing action over the dihydrate or theanhydrous product of the prior art. In addition, it should combine theabove properties in a product which, when used in a dentifrice, does notproduce an unpleasant taste or feel in the mouth.

Anhydrous dicalcium phosphate produced by the methods of the prior arthas a polishing power of about 20 to about 30 which is too high to meetthe current specifications for a satisfactory dental polishing agent.Thus, when this material is used in a dentifrice, it is so highlyabrasive that it has a deleterious effect on the dentine of the teeth.Consequently, there is an urgent demand in the dentifrice art for ananhydrous dicalcium phosphate having a polishing power within the limitsof about 2 to about 15, which also possesses all of the previously knowndesirable properties of the above anhydrous salt.

It is, therefore, the primary object of the present invention to providean anhydrous dicalcium phosphate having a polishing power within thecurrent specifications for a satisfactory dental polishing agent.

An additional object of the invention is to provide an anhydrousdicalcium phosphate which has a polishing power within the limits ofabout 2 to about 15 and an improved cleaning and stain-removing actionover the dihydrate and the anhydrous dicalcium phosphate of the priorart.

A further object of the invention is to provide an anhydrous dicalciumphosphate having a polishing power of from about 2 to about 15 which,when used in a dentifrice, does not produce an unpleasant feel or tastein the mouth.

A still further object of the invention is to provide a method ofproducing anhydrous dicalcium phosphate having the above combination ofproperties.

Other objects and advantages will become apparent to those skilled inthe art as the description of the invention proceeds.

We have made the surprising discovery that by following the methodhereinafter described, anhydrous dicalcium phosphate can be produced ina form that is eminently suitable for use as a dental polishing agent.This new form of anhydrous dicalcium phosphate is a pseudomorph ofdicalcium phosphate dihydrate, i. e., it has the same apparent crystalformation as the dihydrate. Moreover, it has the very desirable propertyof having a polishing power within the range of about 2 to about 15which is well within the present requirements of the dentifrice art.

In accordance with the above method, a slurry of dicalcium phosphatedihydrate crystals having a particle size within the range of about 2 toabout 38 microns is prepared by precipitation, after which the crystalsare separated and converted to the anhydrous state by heating. Inanother embodiment, a slurry of relatively coarse 2,697,024 PatentedDec. 14, 1954 2 crystals of dicalcium phosphate dihydrate is producedand milled so as to reduce the size of the crystals to a value withinthe above limits, whereupon the slurry of fractured crystals is spraydried at a temperature producing anhydrous dicalcium phosphate having anignition loss 3.295 kilograms of lime containing 73% CaO was slurriedwith 6.5 gallons of water and the resulting product added to 5:1kilograms of 61.6% P205 phosphoric acid which had been diluted withwater to a concentration of 23% P205 acid, the lime being added until 10milli liters of the resulting slurry was titratable with 2 millilitersof 0.1507 N NaOH to a phenolphthalein end point. During the aboveoperation, the reaction temperature was maintained at about 45 C. Theslurry of dicalcium phosphate dihydrate thus produced was ball milledand then pumped to a spray drier where it was spray dried using an airinlet temperature of about 625 F. and an air outlet temperature of about260 F. The spray dried product was characterized by having an ignitionloss of 8.8%, a polishing power of 9 and a particle size such that onaddition to a 325 mesh screen only 2.88% by weight of material wasretained.

Example II One gallon of 75% HsPO4 was diluted to 23% P205 acid and thenneutralized with an 8.74% CaO slurry, the resulting reaction temperaturebeing maintained below 45 C. by means of a cooling coil. The slurry ofdicalcium phosphate dihydrate thus produced was removed from the reactorand ball milled for 30 minutes. After the last mentioned operation,phosphoric acid was added until 10 milliliters of the solution wastitratable to a phenolphthalein end point with 1.16 milliliters of0.1212 N NaOH. The slurry was then spray dried using an air inlettemperature of about 600 F. and an air outlet temperaure of about 300 F.The spray dried product on testing by the method hereinafter describedwas found to have a polishing power of 8.5.

Example 111 A dilute lime slurry containing about 8.74% CaO was added toa 23% P205 phosphoric acid solution with agitation to form a slurry ofdicalcium phosphate dihydrate, the lime addition being carried out until1.26 milliliters of 0.1367 N NaOH titrated 10 milliliters of a sample ofthe slurry to a phenolphthalein end point. The above slurry wastransferred to a spraydrier and dried using an air inlet temperature ofabout 800 F. and an air outlet temperature of about 370 F. The spraydried product thus obtained had an ignition loss of 8.3% and a polishingpower of 27.

It is evident from Examples I and II which resulted in the production ofa product having a polishing power within the desired range that millingis an essential step of the process since this step was missing fromExample IIIand the polishing power of the resulting product was toohigh.

Example IV 15.9 lbs. of lime containing 73% CaO was mixed with asufficient amount of water to produce a 8.74% CaO slurry. This slurrywas added to 27.4 lbs. of 75 HaPO4 night. The dried product was thenslurried in water,

,milled in .a ball mill, filtered and heated in a pan for about 36 hoursat 75 C. The product thereby obtained had a polishing power of 8.5.

Example V .The procedure describedin Example IV was repeated except thatthe milling step was omitted. The product thus obtained had a polishingpower of 41.5.

The results obtained by the methods described in Ex- :amples IV and Vfurther illustrate that it is essential ;to

.substantially reduce the particle size of the crystals in order toproduce a product having a polishing power with- .in the desired range.

Example VI tdilute'lime slurry containing 8.74% CaO was added to adilute phosphoric acid solution which was-heated to a temperature of C.,the above solution being prepared by diluting 177 grams of HsPO4 withwater ;to 8."'P2Q5 content of 15% P205. The lime slurry was addeduntil aslurry of dicalcium phosphate was produced 'whose mother liquor titratedto a phenolphthalein end :point with 6 cc. of 0.1 N NaOH per 10 cc. ofliquor.

One half of the slurry of anhydrous dicalcium phosphate crystals wasball milled to a particle size within the range of 2 to 38 microns. Theground crystals were separated from their motherliquor andthen pan driedslowly at a temperature of 70 C. This sample had an ignition loss of8.3% and a polishing power of 25.3.

The other half of the slurry of crystals was separated from its motherliquor, pan dried slowly at 70 C. and

subsequently milled to a particle size within the range of'2 to 38microns. The ignition loss for this sample was L 9% and the polishingpower was 28.1.

The above example demonstrates that if the dicalcium phosphate isinitially precipitated in the anhydrous state and then ground to aparticle size within the limits of 2 to 38 microns, separated andfinally dried, an excessively abrasive product is obtained; it alsoshows that if the :dicalcium phosphate is precipitated in the anhydrousstate, separated, then dried and finally ground to the above particlesize, the product still does not possess the desired low polishingpower.

Incarrying out the method of the instant invention,

a dilute solution or slurry of lime is reacted with dilute phosphoricacid in substantially the proportions calcu- -lated to yield dicalciumphosphate 2H2O, the reaction being carried out at a temperature which ismaintained below about 45 C. More specifically, these materialsarereacted together in the proportions yielding a mother liquorthat-is-titratable to a phenolphthalein. end point with about 1 to about 2milliliters and preferably with not more than 1.5 milliliters of 0.1507N NaOI-I.

The product of the above reaction may be rapidly cooledto yield veryfine crystals having a particle size -Within the range of about 2 toabout 38 microns or it may be crystallized at room temperature and thenconverted .Air inlet temperature About 500 F. to about 1000 F.Preferably about 600 F. to about 800 F. Air outlet temperature About 280F. to about 380 F.

The invention, however, is not limited to these conditions ,as;the inletand outlet temperatures may be varied widely so ,long ;as anhydrousdicalcium phosphate having .an ignition loss of not over 12% isobtained.

Asa modification of the above'described procedure, the relatively coarseslurry of dicalcium phosphate dihydrate crystals may be filtered,centrifuged or treated in any other suitable manner for separating thecrystals which rn ay then be milled to a particle size within thelimits. of -..:a bo,u t.2-to about 38 microns. The milled dicalciumphos- ;;:phate,.dihydrate,is then subjected to slow drying forabout 10to about 48 hours at a temperature of about 65 C.

p to about C. and then for about A hour to about 1 hour at 325 C. toabout 250 C. to convert it to the anhydrous state in which it thenpossesses a polishing power within the desired limits. Morespecifically, the milled dihydrate is heated to a temperature of about75 C. for about 30 hours and then for about /2 hour at 300 C. to convertit into a relatively soft anhydrous product.

holder, moistened and then lowered against the copper strip, the brushbeing adjusted so that it touches the copper strip along the entirelength of the brush. Once this has been achieved, the brush is rigidlyfixed in position.

The sample to be tested is prepared by adding twenty grams of thepowdered material to a 150 milliliter beaker and then mixing therewithglycerine until the resulting paste flows readily upon tilting thebeaker. The paste thus prepared is poured into the above trough and thebrush lowered into contact with the copper strip.

The brushholder is reciprocated 14,400 times in about to 123 minutes sothat including the forward and reverse motions of the brush a total of28,800 strokes are applied to the copper strip.

At the end of the foregoing operation, the copper strip is removed,washed with water and then with ethyl alcohol. The washed strip is thendried for 5 minutes at 60 C., cooled in a dessicator and reweighed. Theloss in weight in milligrams represents the polishing power. Thus, ifthe copper strip losses 10 milligrams in weight, the abrasive materialbeing tested is said to have a polishing power of 10.

The ignition loss as used herein represents the loss in weight of theanhydrous dicalcium phosphate product when heated for /2 hour at 800 C.

The expression about 2 to about 38 microns mentioned earlier herein hasreference to particle size as measured by sedimentation rate andcalculations based on Stokes law of falling spheres [see method ofHinkley, Industrial and Engineering Chemistry (Analytical Edition)volume 14, pp. 10-13, 1952; and also method described by Colbeck andHarner found in industrial and Engineering Chemistry, volume 19, page58, 1927].

The above description and examples are intended to be illustrative only.Any modification or variation therefrom which conforms to the spirit ofthe invention is intended to be included within the scope of the claims.

What we claim is:

1. The method of producing anhydrous dicalcium phosphate having apolishing power within the range of about 2-to about 15, which comprisespreparing a slurry of dicalcium phosphate dihydrate crystals having aparticle size within the range of about 2 to about 38 microns and thenconverting said product to the anhydrous state by heating.

2. The method of producing anhydrous dicalcium phosphate having apolishing power within the range of about 10 to about 13, whichcomprises preparing a slurry of dicalcium phosphate dihydrate crystalshaving a particle size within the range of about 2 to about 38 micronsand then converting said product to the anhydrous state by heating.

3. The method-of. producing anhydrous dicalcium phosphate having :apolishing power within the range of about 2 to.about 15, which comprisespreparing a slurry .of dicalcium phosphate dihydrate crystals having aparticle size within the range of about 2 to about 38 microns and thenspray drying said slurry at a temperature sufficient to produceanhydrous dicalcium phosphate.

4. The-method of producing anhydrous dicalcium phosphate having apolishing power within the range of about 2 to about 15, which comprisespreparing a slurry of dicalcium phosphate dihydrate crystals, millingsaid slurry soas to reduce'the particle size of said crystals to a valuewithinthe-range .of about..2.to ,about,38.microns and then spray dryingsaid product at a temperature producing anhydrous dicalcium phosphate.

5. The method of producing anhydrous dicalcium phosphate having apolishing power within the range of about to about 13, which comprisespreparing a slurry of dicalcium phosphate dihydrate crystals, millingsaid slurry so as to reduce the particle size of said crystals to avalue within the range of about 2 to about 38 microns and then spraydrying said product at a temperature producing anhydrous dicalciumphosphate.

6. The method of producing anhydrous dicalcium phosphate having apolishing power within the range of about 2 to about 15, which comprisespreparing a slurry of dicalcium phosphate dihydrate crystals, millingsaid slurry so as to reduce the particle size of said crystals to avalue within the range of about 2 to about 38 microns and then spraydrying said product under conditions yielding a product having anignition loss not in excess of 12% by weight.

7. The method defined in claim 6 wherein the spray drying operation iscarried out at a temperature producing a product having an ignition lossof from about 8% to about 9% by weight.

8. The method of producing anhydrous dicalcium phosphate having apolishing power within the range of about 2 to about 15, which comprisespreparing a slurry of di calcium phosphate dihydrate crystals, millingsaid product so as to reduce the particle size of said crystals to avalue within the range of about 2 to about 38 microns and thenconverting the resulting product to anhydrous dicalcium phosphate byheating.

9. The method of producing anhydrous dicalcium phosphate having apolishing power within the range of about 2 to about 15, which comprisespreparing a slurry of dicalcium phosphate dihydrate crystals, millingsaid slurry so as to reduce the particle size of said crystals to avalue within the range of about 2 to about 38 microns, separating saidcrystals, heating said separated crystals to a temperature of about 75C. for about hours and then at 300 C. for about /2 hour to convert saidcrystals to the anhydrous state.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,818,114 Carothers et a1 Aug. 11, 1931 2,277,854 Lecoq Mar.31, 1942 2,296,494 Block Sept. 22, 1942

1. THE METHOD OF PRODUCING ANHYDROUS DICALCIUM PHOSPHATE HAVING APOLISHING POWER WITHIN THE RANGE OF ABOUT 2 TO ABOUT 15, WHICH COMPRISESPREPARING A SLURRY OF DICALCIUM PHOSPHATE DIHYDRATE CRYSTALS HAVING APARTICLE SIZE WITHIN THE RANGE OF ABOUT 2 TO ABOUT 38 MICRONS AND THENCOVERTING SAID PRODUCT TO THE ANHYDROUS STATE BY HEATING.